1. Field of the Invention
The present invention relates to heat-curable organopolysiloxane compositions formulated by admixing diorganopolysiloxane resins, fillers, a group of individual additives and organic peroxides.
2. Description of the Prior Art
Heat-curable or vulcanizable organopolysiloxane compositions are well known to this art and have the advantage, on the one hand, of being easy to process in the raw state and, on the other, of providing, after heat-curing, elastomers which are characterized by good physical properties.
The introduction of various additives in order to modify the properties of organopolysiloxane compositions, and of the resultant elastomers, is widely described in the literature, patent or otherwise, the earliest of which date back to the beginnings of the commercialization of heat-cured silicone elastomers.
Thus, U.S. Pat. No. 3,379,607 describes the addition of a compound such as methacryloyloxypropyltrimethoxysilane to a polysiloxane elastomer.
U.S. Pat. No. 4,202,812 describes adding acryloyloxy (or methacryloyloxy)alkylalkoxysilane in a proportion of 0.08 to 0.3 parts of additive per 100 parts of a silicone rubber base combined with 25 to 300 parts of nonreinforcing fillers in order to improve the tensile properties of the vulcanized elastomers.
Published European patent application EP-A-149,882 describes the addition, to an organopolysiloxane elastomer composition, of a mixture of additives which may be an organosilicone compound containing an acryloyl- (or methacryloyl)oxyalkyl group combined with at least two other additives selected from among an organohydropolysiloxane, an organofluoro polymer and a boron-containing compound. Furthermore, at page 18, lines 6 to 12, of this '882 application, it is indicated that this mixture of additives may additionally contain other adjuvants which are well known to the silicone rubber art, especially calcium, magnesium and barium hydroxides and oxides, which neutralize the decomposition products formed during crosslinking or at elevated temperature, as described, for example, in published European patent application EP-A-51,369.
U.S. Pat. No. 3,261,801 describes improving the reversion resistance of silicone rubbers by adding from 0.1 to 10 parts of a barium oxide per 100 parts of organopolysiloxane resin.
In U.S. Pat. No. 3,711,520 the flame resistance of silicone rubbers imparted by the addition of platinum (U.S. Pat. No. 3,514,424) is improved by additionally adding an oxide of an alkaline earth metal or of a Group 2b metal (Zn, Cd, Hg).
Japanese application Kokai No. 48/093,658 describes the conjugate addition to an organopolysiloxane composition of an organohydropolysiloxane and a calcium derivative such as CaO and Ca(OH).sub.2 to improve the residual compression set and to eliminate efflorescence.
U.S. Pat. No. 4,301,056 describes improving reversion resistance, the residual compression set and hot oil resistance by the introduction of calcium hydroxide treated with an organosilicon compound.
Presently, the internal combustion engine automotive industry is in full expansion for silicone elastomers as a result of the increase in the internal temperature of the engines, in order to improve their thermal efficiency and to reduce fuel consumption. Motor vehicle manufacturers are therefore seeking to replace the conventional rubbers which have an inadequate thermal resistance by other rubbers, especially by silicone rubbers.
Therefore, serious need exists in this art for an organopolysiloxane elastomer composition which cures to a silicone rubber which, on the one hand, has and retains, when heated in contact with the various oils employed in an internal combustion vehicle, good mechanical properties, such as residual compression set and good elasticity and, on the other hand, has good resistance to various types of hot oils in contact with the silicone rubber seals in closed systems.